A 63-year-old woman presents with a 6-month history of recurrent epigastric pain and nausea. Abdominal ultrasound reveals a 13-mm hypoechoic lesion in the tail of the pancreas. Physical examination shows flushing of the face, periorbital edema, and hypotension (blood pressure =90/50 mm Hg). Laboratory studies disclose normal serum levels of gastrin, amylase, insulin, and vasoactive intestinal polypeptide. Urinalysis demonstrates elevated levels of metanephrines (10 mg per 24 hours). Which of the following is the most likely diagnosis?